Magnetic,Optical,Electrical And Coupling Properties Of BiFeO₃/CoFe₂O₄ Composite Films

Petroleum is not only an extremely important strategic resource but also closely related to people’s daily life.Petroleum is exploration and exploitation is usually in the wild or desert,which leads to the difficulty of obtaining electricity for its operating equipment.Multiferroic materials with a magnetoelectric coupling effect can collect and convert weak energy（such as mechanical vibration,biological motion,electromagnetic wave,etc.）in the environment into usable energy,which will be expected to solve this problem.Multiferroic materials have single-phase multiferroic materials and composite multiferroic materials.Compared with single-phase multiferroic materials,composite multiferroic materials have attracted much attention due to their easy realization of the strong magnetoelectric coupling effect at room temperature.With the development of electronic information technology,the requirements of miniaturization,lightweight,and integration of devices are getting higher and higher.Two-dimensional thin-film materials have become a hot research topic.By changing the stress between the ferroelectric phase and magnetic phase interface,it is expected to achieve a breakthrough in the magnetoelectric coupling effect.Therefore,Bi Fe O₃（BFO）with a strong magnetostrictive effect and excellent ferroelectric properties is selected as the ferroelectric layer in this paper,and Co Fe₂O₄（CFO）with a good magnetostrictive effect was selected as the magnetic layer to construct the magnetoelectric composite film.The effects of the annealing process,composition and film thickness of the ferroelectric phase,and magnetic properties on the structure and properties of the composite film were studied.（1）Study on annealing process of BFO/CFO thin filmThe defects caused by the volatilization of Bi element and the oxygen vacancy in the BFO film will affect the structure and properties of the film.In this paper,the effects of rapid and slow annealing systems on the structure and properties of the films were studied,and the oxygen vacancy concentration in the films was regulated by changing the annealing atmosphere.The results show that under the rapid annealing system,due to the rapid heating rate,the residual stress inside the film is more.Although the film can be rapidly crystallized,the film grains cannot fully grow and the compactness is poor,resulting in poor performance.However,in the slow annealing system,the heating rate is relatively slow,and the film grains can continue to grow after nucleation.The degree of crystallization is high,and the structure is dense,showing good magneto-optical properties.It is also found that the magnetoelectric coupling effect of BFO/CFO films can be enhanced by the external light field.In addition,by changing the annealing atmosphere of the film,the formation of oxygen vacancies in the film can be effectively reduced,the density of the film can be improved,and the magneto-optical and its coupling properties can be effectively improved.（2）Composition control of magnetic,optical,electrical,and coupling properties of BFO/CFO filmsBy changing the number of deposition layers of the BFO layer and CFO layer,the regulation laws of different compositions on the magneto-optical and coupling properties of BFO/CFO films were studied.The results show that with the increase of BFO composition in the composite film,there is also an interface between the BFO layer and the layer.The dielectric constant of the film gradually decreases with the increase of BFO content,the photoelectric conversion efficiency of the composite film increases first and then decreases with the increase of BFO content.The photoelectric conversion efficiency of the composite film is the highest when the composition is 4BFO/6CFO.With the increase of CFO content,the residual magnetization and saturated magnetization of the film gradually increase.When the CFO layer is composed of more than 6 layers,the ferroelectric properties of the composite films decrease with the increase of leakage current.The magneto-dielectric response of the composite film and the magneto-dielectric response under light field increased first and then decreased with the increase of BFO content.When the composition was 5BFO/5CFO,the magneto-dielectric coefficient of the composite film and the magneto-dielectric coefficient under light field were the largest,2.21 and 3.88,respectively.The change of ferroelectric properties with the external field was further studied.It was found that the magnetoelectric coupling coefficient of5BFO/5CFO film was greatly improved under illumination,reaching 115%.（3）Thickness regulation of magnetic,optical,electrical and coupling properties of BFO/CFO filmsThin film thickness is one of the main factors affecting the structure and properties of thin films.When adjusting the composition of BFO/CFO films,we found that CFO layer showed better compactness than BFO layer.Therefore,in this chapter,the effects of different CFO layer thicknesses on the microstructure and properties of the composite films were studied.The results show that when the CFO layer thickness is above 380 nm,the Fe₃O₄ phase appears.The dielectric constant and maximum photoelectric conversion efficiency first increase and then decrease with the increase of CFO film thickness.With the increase of CFO layer thickness,the remanent polarization strength of the thin film has a certain increase,but when the CFO layer thickness is more than 420 nm,the hysteresis loop of the thin film is affected by the leakage current.When the thickness of CFO layer is 340 nm,the maximum magnetodielectric coefficient is 2.23.When the thickness of CFO layer is 380 nm,the magnetodielectric coefficient of the composite film increases to 2.01 under illumination.At the same time,the change of ferroelectric properties with the external field was further studied.It was found that when the thickness of CFO was 340 nm,the magnetoelectric coupling coefficient of the film increased by 110%under illumination.